The present invention relates generally to an antenna for transmitting and receiving electromagnetic waves carrying messages in the radio frequencies and, in particular, to Global Positioning System (GPS) antennas operating in the 1575.42 MHz frequency band.
A CDMA telephony system is based on spread-spectrum technology and is one of the most widely used digital wireless services today. The spread-spectrum signal requires sophisticated broadcast power management and soft hand-overs between base stations. This means that the base stations must be precisely timed. With the CDMA wireless telephony system, each transmitter must maintain its frequency to within one part in ten billions. It is advantageous and desirable to have an antenna in a mobile phone so as to allow the mobile phone to receive messages in the CDMA as well as WCDMA system. Furthermore, the U.S. Federal Communications Commission has introduced regulations requiring wireless service providers to supply the location of all system users for emergency situations. In particular, the Wireless Communications and Public Safety Act, also known as E-911, makes 911 the universal emergency number for wireless telephones so that the cellular user who has been in an accident can be automatically located. Thus, it is advantageous and desirable to provide a positioning antenna in a mobile phone.
Currently, GPS technology enables accurate timing and synchronization between base stations so that cellular calls can be flawlessly passed from one base station to another. The GPS satellites transmit two microwave carrier signals: the L1 frequency (1575.42 MHz) which carries the navigation message and the Standard Positioning Services (SPS) code signals; and the L2 frequency (1227.60 MHz) which is used for ionospheric delay measurement carried out by the Precise Positioning Services (PPS) equipped receivers. Three binary codes are used to shift the L1 and/or L2 carrier phases: 1) the Coarse Acquisition (C/A) Code, which is a repeating 1.023 MHz Pseudo Random Noise (PRN), code-modulates the L1 carrier phase, spreading the spectrum over a 2.046 MHz bandwidth, or a larger bandwidth such as 10 MHz. For the code-phase modulation, each GPS satellite is assigned a different C/A code PRN, so that each GPS satellite can be identified by a unique PRN code; 2) the Precise (P) Code uses a 10.23 MHz PRN code for modulating both the L1 and L2 carrier phases for the military receivers; and 3) the navigation message, which is used to modulate the L1-C/A or P(Y) code signal, is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, system time, position, clock corrections, and other system parameters.
As described above, GPS provides both the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). Only the SPS is designated for the civil community. Thus, in order to receive signals broadcast from the GPS satellites, the antenna should be tuned to the L1 band with a suitable bandwidth. The GPS antenna is required to be right-handed circularly polarized (RHCP) or right-handed elliptically polarized (RHEP) and to provide near hemispherical coverage in order to achieve optimum performance.
In a mobile phone, a communicator device or other small hand-held electronic device, it is preferable for the GPS antenna to be small in size and rugged. Preferably, the antenna can be mass-produced without individual tuning in order to reduce cost. Currently, a number of small size antennas are used in small electronic devices. U.S. Pat. No. 5,986,609 (Spall) discloses an antenna configured to be enclosed within the flip cover of a radiotelephone and to resonate in three frequency bands including a GPS L1 band. The disclosed antenna is a half-wave dipole antenna which includes two tapered radiating elements located on opposite sides of a substrate. Although the disclosed antenna is intended to be used in a radiotelephone, its implementation is restricted in that the antenna cannot be placed differently to improve the radiation efficiency. Furthermore, the size of the antenna is not small enough to be implemented within the phone body. In an article entitled xe2x80x9cDesign of GPS Microstrip Antenna using Nearly Square Patchxe2x80x9d (1997 Asia Pacific Microwave Conference), Chih-Yu Huang et al. discloses a ceramic patch design. Ceramic patches are known to be compact. However, small ceramic patch antennas are extremely narrow-banded and need to be individually tuned. In the monograph entitled xe2x80x9cAntennasxe2x80x9d (McGraw-Hill 1988), Kraus discusses a quadrifilar helix in which each of the four (xc2xd)xcex wires forms a half-turn of a helix on a cylindrical frame. However, the quadrifilar helical antennas as disclosed are unpractically large for mobile phones and small-sized, hand-held devices.
It is, therefore, an object of the present invention to provide a miniature circularly or elliptically polarized antenna having sufficient gain which can be mounted on or enclosed within a mobile phone or other miniature hand-held electronic device. In particular, the antenna is designed to resonate over the GPS frequency bands or other radio frequency bands with improved radiation efficiency over the existing miniature antennas.
It is another object of the present invention to provide a method of installing miniature antennas in a mobile phone or other hand-held electronic device in order to achieve an improved axial ratio.
According to one aspect of the present invention, the radio frequency antenna comprises a radiating element implemented on a supporting frame. In order to match the impedance of the radiating element, the feed of the radiating element is tapped. Thus, one end of the radiating element is short-circuited to a ground plane and the feed point of the antenna is located at the close proximity of the grounding point. Preferably, the radiating element is substantially equal to a quarter-wave electric length or an odd integral multiple thereof. It is also preferred that the supporting frame be made of a material having a medium relative permittivity so as to effect dielectric loading to the antenna in order to reduce the size thereof. The relative permittivity of the support frame material can range from 2 to 50 in the radio frequency range. Preferably, the relative permittivity of the supporting frame material ranges from 2 to 50 in the GPS frequency range. With this dielectric loading technique, the size of the antenna can be substantially reduced.
According to the preferred embodiment of the radiating antenna, the radiating element has a helical shape spiraling around the supporting frame.
According to another embodiment of the antenna, the radiating element is shaped like a single meander line on a surface of the supporting frame.
The method of receiving signals in the radio frequency range by a mobile phone, according to another aspect of the present invention, comprises the steps of:
providing a radiator part having substantially a quarter-wave electric length to receive electromagnetic waves containing the signals; and
providing means for effecting dielectric loading on the radiator part in order to reduce the size thereof.
In particular, the radiator part includes a signal conduit part joining the radiator part at a feed point in order to retrieve the signals from the radiator part; and an impedance matching part joining the radiator part at the proximity of the feed point in order to match the impedance of the radiator part.
A further aspect of the present invention is a method of achieving a right-handed circularly or elliptically polarized antenna in a mobile phone, wherein the mobile phone has a phone body having four corners to define a plane having a long axis and a short axis. The method comprises the steps of: providing at one of the phone body corners a radiator part having substantially a quarter-wave electric length to generate a radiating mode along the long axis and another radiating mode along the short axis; providing a support frame located at the proximity of the radiator part to effect dielectric loading on the radiator part in order to reduce the size thereof. Furthermore, the radiator part includes a resonating region and a feeding region, wherein the feeding region has a signal conduit part joining the resonating region at a feed point in order to retrieve signals from the resonating region; and an impedance matching part joining the resonating region at the proximity of the feed point.
It should be noted that the radiator part as described above is divided into a resonating region and a feeding region only in a loose sense. The feeding region, to some extent, also affects the resonating properties of the antenna as it also affects the radiating modes of the antenna.
Alternatively, two or more antennas can be implemented on different corners of the substrate so that one antenna can be selected by switching means to achieve improved circular or elliptical polarization.
A further aspect of the present invention is an electronic device having means for receiving signals in the radio frequency range, wherein the receiving means comprises: a radiator part having substantially a quarter-wave electric length implemented on a support frame having a medium relative permittivity for effecting dielectric loading on the radiator part; and means for matching the impedance of the radiator part.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 1-9c.